It is generally known to use aluminum or aluminum alloy substrates as structural members for many applications. The use of aluminum in many applications provides numerous advantages, because it is lightweight, easily handled, and generally inexpensive.
In various applications, however, it is desirable to coat the aluminum or aluminum alloy substrate with a dissimilar metal that is harder than aluminum. For example, it is known to use an aluminum or aluminum alloy substrate to make internal combustion engines with aluminum pistons wherein the aluminum piston or the cylinder bore is coated with another metal that is harder than aluminum to prevent piston skirt scuffing, galling and subsequent engine seizure.
One method for depositing iron coatings onto aluminum or aluminum alloy substrates is by electroplating. One such method for electroplating iron onto substrates containing aluminum is disclosed by U.S. Pat. No. 5,516,419 issued to Phan et al. (hereinafter "Phan"). In the process disclosed by Phan, a bath separate from the electroplating bath is required to activate the aluminum or aluminum alloy substrate. See Phan, Column 2, Lines 48-58. Additionally, after the substrate is activated, another separate bath is required to place a transitory protective layer, such as a zinc layer, onto the activated substrate to prevent aluminum oxides from reforming after the substrate has been activated. See Phan, Column 2, Lines 59-67, and Column 3, Lines 1-2. Finally, in Phan, an undercoating or intermediate layer, such as a nickel layer, is plated onto the substrate prior to plating iron onto the intermediate layer in another separate plating bath. See Phan, Column 3, Lines 3-25. The undercoating layer is required to provide a layer to which the subsequently-plated iron layer will adhere. See Phan, Column 3, Lines 16-22. In essence, the iron is not directly plated to the aluminum or aluminum alloy substrate, but is instead plated to an undercoating layer of a different metal which has in turn been plated onto the aluminum or aluminum alloy substrate.
The use of a method for electroplating as described in Phan has significant shortcomings for high volume commercial production. The use of a separate activation bath, a transitory layer, and a undercoating layer all add to the expense, complexity, and time involved with plating an iron layer onto an aluminum or aluminum alloy substrate. Additionally, the use of these separate steps and separate baths adds the difficulty and expense of disposing of the waste produced in each of these steps and baths.
Another problem that has existed with iron plating baths is that after use, impurities, such as copper or aluminum, remain in the bath solution and adversely affect further plating processes. When impurity concentration becomes too high, the iron plating process must be stopped so that the bath solution can be cleaned or changed.
An electroplating bath and electroplating method which permitted higher throughput before requiring bath cleaning would be highly desirable. In addition, the electroplated product should have good hardness or wear resistance properties.
It is desirable to provide a formulation and method for electroplating iron directly onto an aluminum or aluminum alloy substrate without the need of a separate activation bath, a transitory layer, or an undercoating layer.
It is also desirable to provide for a method and apparatus for purifying an iron plating bath solution to remove impurities from the bath without stopping the plating process to clean or change the bath solution.